go-tour-56.go

package main

import (
    "fmt"
    "math"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
    return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

func Sqrt(x float64) (float64, error) {
    if x < 0 {
        return x, ErrNegativeSqrt(x)
    }
    z := float64(1)
    for i := 0; ; i++ {
        z1 := z
        z = z - (z * z - x) / 2 * z
        if math.Abs(z - z1) <= 0.001 {
            return z, nil
        }
    }
    return z, nil
}